1. Field of the Invention
The present invention relates to thermal expansion arresters utilizing a compressible volume of air to prevent or relieve the build up of pressure in hot water systems.
2. Description of the Related Art
Numerous devices have been developed to relieve the pressure caused by the thermal expansion of water within a hot water system such as is associated with water heaters in most houses and residential buildings. Most residential building codes require that check valves or backwater valves be installed on the incoming water line for buildings to prevent the downstream potable water system from being contaminated if back pressure is created due to a line break or use of fire hydrants. With a check valve on the water inlet and all of the downstream valves or faucets in a residence or building closed, most residential water systems become closed systems. The pressure within such a closed system can increase significantly due to thermal expansion of the water upon heating. In situations in which most of the heated water in the water heater is drained out so that the water heater is filled with relatively cold water, which is then allowed to heat back up to the water heater thermostat set point, the volume of the water in a conventional residential forty gallon water heater can expand by as much as a gallon.
The increased pressure resulting from this expansion in volume can cause damage to or blow out seals on downstream faucets or even cause a rupture in the hot water tank if some form of pressure relief system is not provided. Although water heaters generally incorporate a pressure relief valve to prevent excessive pressure buildups, release of these valves results in a generally uncontrolled release of water from the water heater which can cause damage to surrounding walls or individuals. Further once a pressure relief valve is tripped, it may not seal properly again providing a continuous leak.
Numerous types of expansion chambers have been developed incorporating a volume of air which will compress to allow expansion of the heated water. For example, U.S. Pat. No. 6,418,969 discloses an in-line expansion tank which may be mounted to the incoming cold water supply line (downstream of any check valve) to permit thermal expansion of heated water out of the water heater tank. However, the in-line expansion tank shown would be relatively expensive to manufacture.
It is also known to use devices similar in construction to the water hammer arrester shown in U.S. Pat. No. 4,819,698 upstream or downstream of the water heater to provide a compressible volume of air maintained within a tube by a piston whose outer face is in communication with the water system. Increases in pressure in the water line act against the piston and compress the volume of air maintained within the tube allowing the expanded volume of water to expand into the tube. However, such commercially available devices have limited volumetric capacity, insufficient to accommodate relatively large increases in pressure occurring when the water heater is completely drained and the cold replacement water is heated up to the thermostat set point, comprising a rise of approximately one hundred degrees Fahrenheit. The relatively expensive, thick-wall construction of the tube of these types of devices is necessary to prevent the tube from being dented and creating a stop limiting the range of motion of the piston and the volume into which the heated water can expand.
It is also known to mount compressible bladders within a water heater. For example, U.S. Pat. No. 6,041,742 discloses a water heater having a collapsible bladder suspended in the tank of a water heater to accommodate thermal expansion of the water in the tank. However, these devices must be connected to an air replenishing valve located outside of the tank as the bladder walls are semi-permeable and slowly lose air. The rubber or synthetic bladders used in such systems are also prone to break down or disintegrate over time rendering the bladders inoperable and producing material or particles which can clog downstream faucets, valves and the like. There remains a need for a reliable thermal expansion system for water heaters which is relatively inexpensive to manufacture, resistant to pressure loss, damage and wear and less expensive to install.